With an increase in the recording density of magnetic disks, the distance between the magnetic disk serving as a recording medium and the magnetic head for use in recording of information or playback has become almost nil close to contact therebetween. The magnetic disk is provided over the surface thereof with a carbon protective film or lubricant film for the purpose of diminishing abrasion due to the contact or sliding of the magnetic head thereon or preventing contamination of the disk surface.
The carbon protective film is produced generally by the sputtering process or CVD process. The disk surface is protected with the two films, i.e., the carbon protective film and the lubricant film thereover.
The lubricants generally in use are fluoropolyethers having functional groups. Examples of functional groups are hydroxyl, cyclophosphazene and amino groups (Patent Literature 1, 2). Particularly, lubricants having a phosphazene group are materials having high resistance to decomposition and known as materials for giving high durability to magnetic disks (for example, Patent Literature 1, 2).
In a heat-assisted (thermally assisted) hard disk drive which is presently developed, a technology is employed in which the disk is locally heated by a laser. When a lubricant is used in such circumstance, the functional group moiety such as hydroxyl group, cyclophosphazene group or amino group easily causes decomposition by the heat of the laser. In addition, the fluoropolyether also decomposes by heat. Due to these decomposition, the lubricant on the disk is transferred to a magnetic head (pick up), and it becomes the problem to cause malfunction.
On the other hand, a lubricant containing a compound which has a perfluoropolyether main chain and an aromatic group in the molecular terminal is known as a lubricant which is excellent in durability of magnetic disk, particularly in LUL durability and resistance to alumina (Patent Literature 3). However, only fluoropolyether having —(CF2O)n(CF2CF2O)m-structure (Fomblin structure) in the molecular main chain is concretely disclosed as a compound of Patent Literature 3. This compound is demanded to have still more superior heat resistance as mentioned later (Patent Literature 3).
Patent Literature 1: JP Patent No. 4137447
Patent Literature 2: JP Patent No. 4570622
Patent Literature 3: JP 2009-266360A
An object of the present invention is to provide a lubricant having superior heat resistance which does not cause decomposition under high temperature and is not transferred to a magnetic head, and magnetic disks.